1 Field of the Invention
The present invention relates to a magnetic recording medium useful in, for example, the memory of an information-related instrument. The present invention further relates for a manufacturing process thereof.
2. Prior Art
With the emergence of a highly information-oriented society, a greater demand has risen recently for the memory of information-related instruments, including computers, to attain a higher recording density. There has been some improvements in both the performance of a recording head to read in and out a magnetic recording medium, as well as in the recording density of a medium in a magnetic recording device.
Dense recording in the medium assumes an increase in a ratio of reproduction signals to medium noises when recorded signals are reproduced.
In general, a conventional magnetic recording medium includes a non-magnetic substrate, a non-magnetic ground layer, a magnetic recording layer, and a protecting layer. The magnetic recording layer records information. The protecting layer protects the magnetic recording layer from abrasion caused from sliding with a magnetic head. A non-magnetic ground layer, a magnetic recording layer, and a protecting layer are formed consecutively on the non-magnetic substrate, one on top of another. The non-magnetic substrate is preferably made from an aluminum alloy or glass. The non-magnetic ground layer controls the crystal orientations of the magnetic recording layer that is deposited thereon.
Referring to FIG. 1, a typical chart shows the layer constitution of a conventional magnetic recording medium. A ground layer 2 is usually formed on a non-magnetic substrate 1, such as glass and aluminum alloy plated with Nixe2x80x94P. The material used for ground layer 2 includes a chromium and a chromium alloy thin film. Further, a magnetic recording layer 3 formed on ground layer 2 employs a magnetic thin film composed of a cobalt and chromium alloy doped with several kinds of elements. A thin film, mainly composed of carbon, is used for an uppermost protecting layer 4.
The layered structure of the medium is conventionally formed with a sputtering technique. This is the method of choice since the sputtering technique facilitates the control of layer characteristics and enables the manufacture of a high-quality thin film.
Magnetic recording layer 3, formed with the sputtering technique on ground layer 2, is a polycrystal consisting of minute grains of cobalt alloy. In the same way as magnetic recording layer 3, ground layer 2 is a polycrystal consisting of minute grains of chromium metal or chromium alloy. Magnetic recording layer 3 grows epitaxially on ground layer 2, with crystallographic orientation of magnetic recording layer 3 coordinated with that of ground layer 2. Magnetic recording layer 3, on the one hand, is usually made of a cobalt and chromium alloy doped with an element, such as platinum and tantalum, that has larger metal bond radius than cobalt. Hence, it is usual that the cobalt alloy has higher lattice constant than pure cobalt metal. On the other hand, ground layer 2 was once made of pure chromium metal for its excellent crystallinity and low cost. However, pure chromium metal has smaller lattice constant than cobalt alloy used in a magnetic recording layer, adversely affecting the epitaxial growth thereof. Then, it is a common practice of late to use chromium alloy doped with an element, including molybdenum, tungsten, vanadium, and titanium, that has a larger metal bond radius than chromium to give ground layer 2.
This practice enlarges the lattice constant of the ground layer crystal and facilitates the epitaxial growth of a magnetic recording layer on a ground layer. Further, Japanese Laid-Open Patent Publication No. 7-21543 discloses a magnetic recording medium wherein the ground layer thereof has a layered structure of a chromium metal and a binary chromium alloy layer. This enables the potentiality for epitaxial growth to consist with required grain growth.
As mentioned previously, a magnetic recording layer is made of a polycrystal consisting of minute crystal grains of a cobalt alloy. In order to form a minute recording bit in the magnetic recording layer, it is desired that the crystal grain dimensions of the cobalt alloy in the magnetic recording layer be more minute and uniform. Achieving this requires control of the thin ground layer located under the magnetic recording layer in terms of a crystallographic property.
A pure chromium ground layer in a magnetic recording medium facilitates the formation of minute crystal grains in the magnetic recording layer. However, differences in the lattice spacing of pure chromium and cobalt alloy crystal set forth earlier provides no increased SNR. A magnetic recording medium with a chromium alloy ground layer, and magnetic recording medium with the layered ground layer consisting of a chromium metal and binary chromium alloy layers disclosed in Japanese Laid-Open Patent Publication No. 7-21543 has the following problems. That is, conventional magnetic recording mediums have been successful in reducing the crystal grain dimensions of a magnetic recording layer to some extent. However, there has been difficulty in sizing the crystal grains in a sufficiently uniform manner, leaving little room for an improvement in SNR.
It is an object of the present invention to provide a magnetic recording medium which overcomes the foregoing problems of the conventional designs.
It is a further object of the present invention to provide a magnetic recording medium with higher recording density.
It is another object of the present invention to provide a magnetic recording medium with an increased SNR.
It is still another object of the present invention to provide a method for making such a magnetic recording medium.
Briefly stated, the present invention provides a magnetic recording medium having a non-magnetic ground layer, a non-magnetic recording layer, a protecting layer formed sequentially on a non-magnetic substrate, one on top of another. A double-layered non-magnetic ground layer consists of a pure chromium layer, positioned closer to a substrate, and a chromium-molybdenum-tungsten ternary alloy layer, positioned closer to a magnetic recording layer. This provides the non-magnetic recording layer formed on such a ground layer with cobalt alloy grains having uniform dimensions. The resulting magnetic recording medium has an excellent SNR.
According to an embodiment of the present invention, there is provided a multi-layered magnetic recording medium having at least three thin films comprising a non-magnetic substrate, a non-magnetic ground layer, a magnetic recording layer, and a protecting layer. The non-magnetic ground layer, magnetic recording layer and protecting layer are formed sequentially on the non-magnetic substrate, one on top of another. The non-magnetic ground layer is double-layered, including a first non-magnetic metal layer and a second non-magnetic metal layer, the first non-magnetic metal layer having a composition different from the second non-magnetic metal layer. The first non-magnetic metal layer, being a pure chromium layer, is positioned closer to the non-magnetic substrate, and the second non-magnetic metal layer, being a chromium-molybdenum-tungsten ternary alloy layer, is positioned closer to the magnetic recording layer.
According to another embodiment of the present invention, there is provided a manufacturing process for a magnetic recording medium including the steps of (a) forming at least a pure chromium ground layer, a chromium-molybdenum-tungsten ternary chromium alloy ground layer, and a magnetic recording layer consecutively on a non-magnetic substrate, one on top of another with a sputtering technique; and (b) forming a protecting layer with one of a sputtering technique and a CVD technique.
A new layer composition for a magnetic recording medium according to the present invention provides cobalt alloy crystal grains in a magnetic recording layer with more uniform diameters with other characteristics intact. This improves the SNR of the magnetic recording medium.
The inventors have devoted themselves to the study to solve the problems set forth earlier. A non-magnetic ground layer was double-layered. The layer closer to a substrate was made of pure chromium metal. Further, the other of the two layers, positioned closer to a magnetic recording layer, was made of chromium-molybdenum-tungsten ternary alloy. This experiment has shown the possibility that the cobalt alloy magnetic recording layer with more uniform crystal grain dimensions is formed on the ground layer.
The present invention was performed in accordance with these findings. The following describes the characteristics of a magnetic recording medium of the present invention. The magnetic recording medium of the present invention comprises a non-magnetic substrate, a non-magnetic ground layer, a magnetic recording layer, and a protecting layer. The non-magnetic ground layer and the magnetic recording layer are formed consecutively on the non-magnetic substrate, one on top of another. The non-magnetic ground layer and the magnetic recording layer are formed with either a sputtering technique of a CVD technique. The protecting layer is consecutively formed on those layers with a sputtering technique. Thus, the magnetic recording medium of the present invention has the layered structure of at least three thin films. The non-magnetic ground layer has a layered structure consisting of two non-magnetic metal layers of different compositions. In addition, the non-magnetic metal layer closer to the substrate is made of pure chromium. The non-magnetic metal layer closer to the magnetic recording layer is made of chromium-molybdenum-tungsten ternary alloy.
The manufacturing process of the present invention comprises the steps of forming at least a pure chromium ground layer, a ternary chromium alloy ground layer consisting of chromium, molybdenum, and tungsten, a magnetic recording layer, and a protecting layer consecutively on a non-magnetic substrate, one on top of another. The pure chromium ground layer, the ternary chromium alloy ground layer, and the magnetic recording layer are formed with a sputtering technique. Protecting layer is formed with either a sputtering technique or a CVD technique. Thus, a magnetic recording medium with a high SNR is manufactured.